Fuel ethanol is currently produced from feedstocks such as cornstarch, sugar cane, and sugar beets. However, the production of ethanol from these sources cannot expand much further due to limited farmland suitable for the production of such crops and competing interests with the human and animal food chain. Finally, the use of fossil fuels, with the associated release of carbon dioxide and other products, in the conversion process is a negative environmental impact of the use of these feedstocks.
The possibility of producing ethanol from cellulose-containing feedstocks such as agricultural wastes, grasses, and forestry wastes has received much attention due to the availability of large amounts of these inexpensive feedstocks, the desirability to avoid burning or landfilling cellulosic waste materials, and the cleanliness of ethanol as a fuel compared to gasoline. In addition, a byproduct of the cellulose conversion process, lignin, can be used as a fuel to power the cellulose conversion process, thereby avoiding the use of fossil fuels. Studies have shown that, taking the entire cycle into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The cellulosic feedstocks that may be used for ethanol production include (1) agricultural wastes such as corn stover, wheat straw, barley straw, canola straw, oat straw, and soybean stover; (2) grasses such as switch grass, miscanthus, cord grass, and reed canary grass, (3) forestry wastes such as aspen wood and sawdust, and (4) sugar processing residues such as bagasse and beet pulp.
Cellulose consists of a crystalline structure that is very resistant to breakdown, as is hemicellulose, the second most prevalent component. The conversion of cellulosic fibers to ethanol requires: 1) liberating cellulose and hemicellulose from lignin or increasing the accessibility of cellulose and hemicellulose within the cellulosic feedstock to cellulase enzymes, 2) depolymerizing hemicellulose and cellulose carbohydrate polymers to free sugars and, 3) fermenting the mixed hexose and pentose sugars to ethanol.
Among well-known methods used to convert cellulose to sugars is an acid hydrolysis process involving the use of steam and acid at a temperature, acid concentration and length of time sufficient to hydrolyze the cellulose to glucose (Grethlein, 1978, J. Appl. Chem. Biotechnol. 28:296–308). The glucose is then fermented to ethanol using yeast, and the ethanol is recovered and purified by distillation.
An alternative method of cellulose hydrolysis is an acid prehydrolysis (or pre-treatment) followed by enzymatic hydrolysis. In this sequence, the cellullosic material is first pre-treated using the acid hydrolysis process described above, but at milder temperatures, acid concentration and treatment time. This pre-treatment process is thought to increase the accessibility of cellulose within the cellulosic fibers for subsequent enzymatic conversion steps, but results in little conversion of the cellulose to glucose itself. In the next step, the pre-treated feedstock is adjusted to an appropriate temperature and pH, then submitted to enzymatic conversion by cellulase enzymes.
The hydrolysis of the cellulose, whether by acid or by cellulase enzymes, is followed by the fermentation of the sugar to ethanol, which is then recovered by distillation.
The temperatures typically used for acid hydrolysis or pre-treatment correspond to saturated steam pressures of 160 psig to 665 psig. The addition of sulphuric acid improves the digestion of the cellulose and shortens the time for pre-treatment from 5–30 minutes to 0.1–5 minutes. Achieving and maintaining these conditions requires a highly pressurized, acid-resistant system. U.S. Pat. No. 4,461,648 (Foody) describes equipment and conditions used in steam explosion pre-treatment, in which the feedstock, steam, and sulfuric acid are added to a reaction vessel, known as a steam gun. In the steam gun, steam is added and the steam pressure is increased rapidly to the desired pressure, followed by sudden explosive decompression. It produces a pre-treated material that is uniform, has most of the hemicellulose hydrolyzed to simple sugar, and requires less cellulase enzyme to hydrolyze the cellulose than other pre-treatment processes.
U.S. Pat. No. 4,461,648 (Foody) teaches that the feedstock materials are fed to the steam gun in a loose, divided form; that is, cut into small pieces and loosely packed. The use of wood chips from commercial chippers or straw cut into uniform pieces of 2–3 inches, as feedstocks is disclosed. Wood chips are loaded into the steam gun at a density of 13 pounds of solids (dry basis) in 1.2 cubic feet, or 10.83 pounds per cubic foot (174 kg/m3). The solids loading of straw or grass in a steam gun is less, typically about 3 pounds per cubic foot.
The use of small, loosely packed pieces assists in penetrating the material uniformly with steam and dilute sulfuric acid. However, the use of such small, loosely packed pieces limits the amount of material that can be loaded into a given volume of steam gun. This increases the number or total volume of steam guns in a plant, which increases the overall cost of steam guns, which may reach the point of impracticality in process complexity and control. More importantly, cutting of the feedstock into small pieces requires power and chopping equipment, adding significantly to the cost of the process.
U.S. Pat. No. 4,136,207 (Bender) teaches steam pre-treatment of sawdust or wood chips measuring up to 4 inches, to produce a ruminant feed. The feedstock is saturated with moisture and compacted at 2000 psi to remove air and improve the subsequent penetration of steam. A rotating helical feed screw conveys the feedstock into a barrel, and steam is fed into the reactor barrel at 200–310 psi. The feedstock proceeds through the barrel, at the end of which is a valve to allow steam and volatiles to escape, and a product valve for treated solids to exit.
U.S. Pat. No. 5,366,558 (Brink) describes a continuous acid hydrolysis process of wood chips, ground forest waste, or other agricultural materials that occurs in several stages. The first stage is a steam treatment in the absence of acid. The material is then mechanically disintegrated to a very small particle size, acidified, and sensitized with oxygen. The sensitized material is then heated with steam for the final hydrolysis reaction. The material is washed countercurrently and the sugar stream and lignin are the products.
U.S. Pat. No. 4,237,226 (Grethlein) teaches a continuous pre-treatment system to enhance the enzymatic or acid hydrolysis of cellulose in oak wood chips. A 5% to 10% solids slurry is moved by a screw or positive displacement moving cavity pump. The slurry is heated to the reaction temperature and a concentrated stream of sulfuric acid is injected, resulting in a final acid concentration of 0 to 1% in the aqueous phase. The hot, acidified slurry is then held at the reaction temperature for the desired time of 1 minute or less. Rapid cooling quenches the reaction by flashing across an orifice or capillary at the outlet to the reactor. Grethlein uses wood chips that have been ground to pass through 60 mesh. In a similar process, U.S. Pat. No. 4,556,430 (Converse) includes a nonaqueous carrier in the feedstock system to decrease the amount of water present. Converse's system uses particulate matter, for example Wilner #247 wood chips or wood flour.
U.S. Pat. No. 5,424,417 (Torget) describes the pre-treatment of feedstock particles by flowing a hot acid or alkali stream through a bed of the particles. For ground wood, the preferred particle size is 0.1 mm to 30 mm. The particles are treated in the reactor at a solids concentration of 5% to 50%.
These pre-treatment processes use small, loosely packed materials, and suffer from the shortcomings associated with high power and equipment requirements for chopping, as well as a large volume requirement. Hence, high costs for pre-treatment.
U.S. Pat. No. 6,557,267 (Wanger) teaches a method of conditioning cotton with steam so as to improve fabric quality and reduce the presence of biological contaminants. A pressed cotton bale is placed in a closed container, from which air is evacuated to a reduced pressure of 50 to 200 mbar; steam is then introduced and permeates the bale for about 5 to 15 minutes, allowing the internal temperature of the bale to reach 60 to 80° C. The container is evacuated, and the procedure is repeated for a minimum of 4 cycles. However, treatment of this type has not been widely accepted in the treatment of lignocellulosic feedstock in ethanol production, as the temperatures are too low for an effective pretreatment.